drm/amd/display: Fix pointer dereferenced before checking

The pointer edid_buf is dereferencing pointer edid before edid is being
 null checked. Fix this by assigning edid->raw_edid to edid_buf only if
edid is not NULL, otherwise just NULL.

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>